Flash memory is classified as non-volatile memory because a memory cell in the flash memory can retain the data stored in the memory cell without periodic refreshing. Most prior art flash memory can store a single bit in a memory cell. In other words, the memory cell can either store a "one" or a "zero". Multi-level flash memory can store two bits per memory cell.
Multi-level flash memory is becoming more popular because of its advantages. In particular, multi-level flash memory lowers the cost per bit for non-volatile memory storage. Further, multi-level flash memory also allows for higher density memories because each memory cell can store two or more bits of data.
Prior art multi-level flash memory has suffered from the problem of difficulty in controlling the data level in the memory cell. Complex electrical circuits are needed to control the program and erase data level of these prior art memory cells. The most difficult aspect is that the data level will shift after cycling tests. What is needed is a multi-level flash memory cell design that is easily written to and read from and is easy to manufacture.